This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Riboswitches are regulatory RNAs that recognize specific small molecules, usually key metabolites, and "switch" downstream gene expression on or off at either the transcriptional or translational level. The discovery of these short cis- acting RNA elements has drastically changed our understanding of genetic regulatory mechanisms. Riboswitches are especially prevalent in Gram-positive bacteria, exemplified by Bacillus subtilis as a model organism, but are also found to control essential genes in important pathogens such as Bacillus anthracis, Staphylococcus, Enterococcus, Streptococcus, Listeria, Clostridium, and Mycobacterium. This and other characteristics have attracted increasing attention to riboswitch- mediated regulation. The three distinct classes of S-adenosyl methionine (SAM) riboswitches are the most commonly found riboswitch classes in nature. These RNAs represent three independent evolution solutions to achieve specific SAM recognition. We seek to determine the crystal structures of these SAM riboswitches in different functional states and bound to different SAM analogs in order to understand their ligand-induced gene regulatory mechanisms.